The use of lead salt materials, such as lead sulfide (PbS), lead selenide (PbSe), and lead telluride (PbTe), in photoconductive and photovoltaic applications is well known in the art. Lead salt materials have band gap energies which allow the absorption of radiation in the infrared spectrum. In photoconductive applications, the absorption of infrared radiation by the lead salt material provides a change in its conductivity. The change in the conductivity can be sensed by sensing a current flowing therethrough. In this way, the lead salt material can be used to sense incident radiation. In photovoltaic applications, the absorption of infrared radiation in the lead salt material provides a potential difference. The potential difference can be used to provide electrical power. Accordingly, lead salt materials can be used in optoelectronic devices such as infrared photodetectors, solar cells, and thermoelectric devices, among others.
Typically, lead salt materials are deposited on a substrate, such as a silicon substrate, by evaporation or chemical bath deposition. However, these deposition methods have several problems. One problem is that the deposited lead salt material may not adhere to the substrate properly. This is particularly a problem if the substrate is silicon. If the lead salt material does not adhere properly, then the yield of devices is low, which increases the costs.
Another problem is that it is difficult to control the composition of the deposited lead salt material. As a result, the composition of the lead salt material tends to be different from one deposition to another. This is further complicated because the composition can undesirably change with time after it is deposited and exposed to the outside atmosphere. The electrical and/or optical properties of the lead salt material depends on the composition, so if the composition changes then these will too.
A further problem is that it is typically desired to sensitize the lead salt material. After it is sensitized, the lead salt material is sensitive to incident IR radiation at higher temperatures, such as room temperature, in comparison to the typical cold temperatures used. Sensitization is usually done by exposing the lead salt material to oxygen. The sensitization can be characterized by measuring the resistivity of the lead salt material. However, the sensitization of lead salt material regions using conventional methods often leads to undesirable differences in resistivity from one lead salt material region to another.
These problems limit the usefulness of any devices formed with lead salt materials fabricated using conventional deposition systems and methods. Hence, there is a need for better systems and methods for depositing lead salt material regions onto substrates.